Because the DSL modem is also bundled with a Router/Firewall. Essentially, your router get an private IP address (192.168.1.x or something like that) from the DSL modem. That's what I don't like about it. The ISP is imposing their Firewall onto your private network. The result is that you got an extra hop between your PC and the internet. I suppose its no big deal performance wise. But I'd prefer to handle the security myself and have one less hop to the Net.